Diamine compounds having sulfonic acid groups or derivatives of sulfonic acid group (sulfonic acid group-containing diamine compounds) are widely used as a crosslinking agent or a raw material for thermosetting polymers or polycondensation polymers such as polyamide, polyimide, epoxy resin, and polyurethane. Fluorene has a fused ring structure composed of three rings, and has a high planarity of the molecular structure. Polymers formed using a sulfonic acid group-containing diamine compound having a fluorene skeleton as a raw material or a crosslinking agent are expected to have properties derived from the high planarity of the fluorene skeleton.
The carbon atom located at the 9-position in the fluorene skeleton is a carbon atom of methylene group, and it thus has a higher reactivity compared to other carbon atoms in the skeleton. Therefore, conventionally, a number of sulfonic acid group-containing diamine compounds (see Patent Literature 1) in which a substituent having an amino group is bonded to the carbon atom at the 9-position in the skeleton, such as 9,9-bis(3,5-dimethyl-4-aminophenyl)fluorene-2,7-disulfonic acid, 9,9-bis(3-methoxy-4-aminophenyl)fluorene-2,7-disulfonic acid, and 9,9-bis(3-fluoro-4-aminophenyl)fluorene-2,7-disulfonic acid, are synthesized and commercially available. Hereinafter, the 1 to 9-positions in the fluorene skeleton may be abbreviated simply as “the 1-position” to “the 9-position”, respectively, by omitting the phrase “in the fluorene skeleton”.